In the autumn of his first year of middle school, a boy named Arata Kouda encounters a strange, large, bird-like creature by the roadside. The creature, who calls himself Kujima, is clearly hungry, so Arata decides to bring him home. The Kouda household is already in a state of quiet tension due to Arata’s older brother, Suguru, who has become a shut-in after failing his university entrance exams. Taking advantage of the family’s distraction, Kujima quickly ingratiates himself and declares he will be staying as a freeloader until winter passes and the warmth of spring arrives.

Kujima is a mysterious being. He can speak human language, claims to have migrated from Russia, and is only three years old. While he resembles a giant penguin to some, his true nature is kept ambiguous, ranging from an alien to a mythical creature. Despite his imposing height of over 180 centimeters, he has a childish and eccentric personality. He fills his days with loud morning screams, terrible cooking attempts, and studying Japanese proverbs, much to the chagrin of the reclusive Suguru. Arata, a kind but slightly lonely boy, becomes Kujima’s primary caretaker and friend, while their mother Miyoshi is welcoming and their father Masaomi is largely amused.

The central conflict arises from Kujima’s need to keep his existence a secret. His Russian stepfather, Maxim, warned him that discovery would bring trouble to their flock. As rumors of a ghost spread through the neighborhood after residents spot Kujima sleeping with ducks or wandering around, Arata must devise a plan, such as passing the creature off as a person in a costume. They eventually confide in a trusting elderly neighbor named Mitsuki-san after a frightening encounter. The story also explores Suguru’s depression, as Kujima’s annoying yet persistent presence gradually chips away at the older brother’s isolation, forcing him back into the world of the living.

Notable narrative arcs involve Kujima learning about human customs through trial and error. When Arata’s childhood friend Makoto, the granddaughter of Mitsuki-san, comes to investigate, she and Kujima immediately clash, leading to a misunderstanding about thumb wrestling versus handshakes. There is also a bittersweet undercurrent regarding Kujima’s limited stay, the mystery of why Maxim sent him away, and the creature’s desire to create lasting memories with the Kouda family through photo albums and shared meals. The story blends slice-of-life comedy with moments of genuine melancholy as this temporary, magical cohabitation moves toward its inevitable spring deadline.